1. Field of the Invention
This invention relates to an image forming apparatus employing an electrophotographic system and also relates to an intermediate transfer member used therein. More particularly, it relates to an image forming apparatus in which a toner image formed on a first image bearing member is transferred once onto an intermediate transfer member (primary transfer) and thereafter further transferred onto a second image bearing member (secondary transfer) to obtain an image, and also relates to such an intermediate transfer member.
2. Related Background Art
Compared with image forming apparatus in which images are transferred from a first image bearing member onto a second image bearing member stuck or attracted onto a transfer drum (Japanese Patent Application Laid-Open No. 63-301960, etc.), image forming apparatus making use of an intermediate transfer member have such advantages that processing or control (for example, holding by a gripper, adsorbing or providing curvature) is not necessary for a transfer material which is the second image bearing member, and hence, the second image bearing member can be selected from various kinds of materials including both thin and thick paper having a thickness from 40 g/m.sup.2 to 200 g/m.sup.2 such as envelopes, postcards and label paper, whether their width is wide or narrow and their length is long or short.
Because of the aforementioned advantages, color copying machines and color printers using an intermediate transfer member have already been put on the market.
However, when in practice such image forming apparatus using an intermediate transfer member are repeatedly used, creating the following problems which must be solved.
For example, since a transfer efficiency from a photosensitive member which is the first image bearing member is not sufficiently high, a large quantity of transfer residual toner (toner remaining after transfer) must be removed by cleaning, and it is essential for the photosensitive member or intermediate transfer member to be provided with a cleaning device, so that the apparatus must have a greater load and also the cleaning device is fairly complicated in its structure and expensive.
As a countermeasure for such problems, e.g., Japanese Patent Application Laid-Open No. 6-222686 discloses a technique in which an intermediate transfer member is used which has on a base layer comprised of an elastic material such as a rubber or an elastomer a surface layer comprised of at least one resin having good release properties wherein the base layer can ensure a uniform and sufficient nip at the time of primary transfer and secondary transfer and the surface layer can achieve an improvement in the transfer efficiency (in particular, secondary-transfer efficiency). In addition, Japanese Patent Application Laid-Open No. 8-50419 discloses an intermediate transfer member in which its outermost layer is made to have a resistivity higher than its lower layer in order to simultaneously improve the transfer efficiency and image quality; and, e.g., Japanese Patent Application Laid-Open No. 8-202064 discloses an intermediate transfer member in which a surface-treated layer having a certain volume resistivity is incorporated with organic resin particles.
However, if the resistivity of an intermediate transfer member is adjusted for an electrical force to hold toner securely enough so that the transferred toner does not scatter in any environment, then the transfer residual toner is hard to clean, or for reasons of molecular structure, contraction and expansion properties and smoothness are remarkably lost, and hence, cracks in the surface layer may occur because of a contact pressure exerted by the member coming into contact with the intermediate transfer member or because of a stretch tension produced when an intermediate transfer belt is used, so that blank areas caused by poor transfer may occur at areas corresponding to the cracks. A frictional force increases between the intermediate transfer member surface and the member coming into contact with the intermediate transfer member surface, so that the rotation of the intermediate transfer member may pulsate to cause color drift when respective color toner images are superimposed. These all cause faulty images. Accordingly, further improvements have been required.